Pattern of Exotherm and Electrolyte Leakage Measured at High Frequency of Satsuma Mandarin Exposed to Subfreezing Temperatures

Abstract

<jats:p>The current study was conducted to relate ice formation to the pattern and rate of leaf and stem injury of Satsuma mandarins on trifoliate orange rootstock. Potted trees were unacclimated, moderately acclimated or fully acclimated by exposing trees to 32/21 °C, 15/7 °C or 10/4 °C, respectively. Freezing treatments consisted of decreasing air temperature at 2 °C·h<jats:sup>-1</jats:sup> until ice formed as evidenced by exotherms determined using differential thermal analysis of stems. Air temperature was then decreased, held constant, or increased and held constant to simulate severe, moderate and mild freeze conditions, respectively. All treatment exhibited exotherms at -2 to -4 °C, which were smaller with milder freezing treatments. Only the fully acclimated trees exhibited multiple exotherms. Leaf watersoaking, an indication of ice formation, occurred concurrently with stem exotherms except for fully acclimated trees where there was up to a 30-min delay and which corresponded with the second exotherm. Electrolyte leakage of leaves began to increase near the peak of the stem exotherm, but increased more slowly with milder freezing temperature treatments. In some treatments, electrolyte leakage reached a plateau near 50% but leaves survived. Leaves died when whole-leaf electrolyte leakage exceeded 50%. These data are discussed within the framework of a proposed mechanism of injury of Satsuma mandarin leaves by subfreezing temperatures, especially multiple exotherms of fully acclimated trees, and the plateau of electrolyte leakage of leaves at the critical level for survival.</jats:p>

A Temperature Index Model to Estimate Long-term Freeze-risk of Satsuma Mandarins Grown on the Northern Coast of the Gulf of Mexico

“…The freezes of 1990-91 and 2000-01 caused no visible injury and had differences in T c and minimum air temperatures of 0.3 and 0.4 °C, respectively. These freezes were of suffi ciently short duration and temperatures mild enough, compared to T c , that the injury was not lethal (Ebel et al, 2004b).…”

“…The upper limit of the model should correspond to injury that typically occurs for nonhardened leaves, which occurs at higher temperatures than cold-hardened leaves (Nesbitt et al, 2002). Injury occurs within 30 min after ice begins to form, which is about -4 °C for nonhardened satsuma mandarin leaves (Ebel et al, 2004b). In both cold-hardened and nonhardened citrus trees, one or a few sites promote crystallization of water, and additional water crystallizes to the ice creating an ice/water front that spreads throughout the entire canopy at rates up to 60 cm•min -1 (Yelenosky, 1991b).…”

“…Slight freezes would cause some leaf damage, although in some cases no injury would be visible. Some slight freezes can cause ice formation and injury as evidenced by leaf water soaking and increased electrolyte leakage, but the leaves survive after thawing and exhibit no visible injury (Ebel et al, 2004b;Young and Peynado, 1967). Slight freezes are economically unimportant since the slight leaf damage would not deter productivity, although they may enhance susceptibility to subsequent freezes (McCown, 1958;Yelenosky et al, 1984).…”

Water Relations of Well-watered Citrus Exposed to Cold-acclimating Temperatures

“…Studies have typically concluded that citrus develop plant water deficits during winter (Ebel et al, 2004;Thomashow, 1999;Yelenosky, 1975Yelenosky, , 1982Yelenosky, , 1985. Most of these studies were conducted without adequate knowledge of soil moisture content or were conducted in dry soils that imposed drought stress.…”

“…There are several lines of evidence that would suggest that at least some changes in plant water deficits occur as a result of cold temperatures and not dry soil. Stomatal aperture and leaf water potential are typically reduced even on days of very low evaporative demand and after the trees have been fully irrigated (Ebel et al, 2004;Yelenosky, 1982).…”